The human spine includes a plurality of vertebra that are spaced from each other so as to define an intervertebral space. For instance, referring to FIG. 1A, intervertebral spinal discs are typically disposed in an intervertebral space 24 defined between a superior vertebra 22a and an inferior vertebra 22b that is adjacent the superior vertebra 22a in the human spine 20. Each healthy intervertebral spinal disc forms a cartilaginous joint that allows for slight movement of the superior vertebra 22a relative to the inferior vertebra 22b, and acts as a ligament to hold the vertebrae 22a and 22b together.
Normal anatomical motion of the spine is partially defined by various soft tissue, including muscles, tendons, and ligaments along with the anatomical structure of the superior vertebra 22a and the inferior vertebra 22b. For instance, when a person moves his or her body, muscles exert pressure on the vertebrae 22a and 22b, thereby causing them to move. The spine 20 defines a central axis 9 that corresponds to the intersection of the medial-lateral and the anterior-posterior planes. In the cervical region, the superior vertebra 22a typically rotates about a medial-lateral axis 3 of the inferior vertebra 22b, an oblique axis 2 that is angularly offset and non-perpendicular with respect to the medial-lateral axis 1, or about both axes 1 and 2.
When a person bends his or her forward, such as to look at their toes, or backwards, such as to look at the sky, the spine 20 undergoes a motion known as flexion and extension, respectively. When the cervical region of the spine undergoes pure flexion and pure extension, the superior vertebra 22a rotates about the medial-lateral axis 3, and thus moves in the sagittal plane.
When a person bends his or her head from side-to-side, the cervical region of the spine experiences a motion known as lateral bending. When a person turns his or her head to the left or right about the central axis 9, the cervical region of the spine experiences a motion known as axial rotation. Each of the vertebrae 22a and 22b defines a pair of facets that engage each other so as to define a respective pair of facet joints. Lateral bending of the cervical region of the spine (for instance at the lower cervical motion segments C3-C7) causes the superior vertebra 22a to move relative to the inferior vertebra 22b. The geometry of the spine 20 in the cervical region dictates that the superior vertebra 22a move relative to the inferior vertebra along a direction that is substantially planar with respect to the facet joints, so that the facets do not interfere with each other during normal anatomical movement. Thus, a mode of motion of the superior vertebra 22a other than flexion-extension causes the superior vertebra 22a at the lower cervical motion segments to undergo a combined motion with respect to the inferior vertebra 22b. For instance, axial rotation of the superior vertebra 22a relative to the inferior vertebra 22b also induces lateral bending of the superior vertebra 22a relative to the inferior vertebra 22b. Similarly, lateral bending of the superior vertebra 22a relative to the inferior vertebra 22b also induces axial rotation of the superior vertebra 22a relative to the inferior vertebra 22b. 
Over time, general wear and tear can cause spinal discs to can become damaged or dislocated giving rise to a problem commonly referred to as a “slipped disc”. In the past, damaged discs were treated by removing the disc and packing the space with bone chips to promote fusion of the adjacent vertebral body. However, this method resulted in a loss of mobility in the patient's lower back. More recent solutions for treating damaged discs include the replacement of the damaged disc with an articulating prosthetic disc implant that permits relative motion between the adjacent vertebral body. Because surgical procedures that replace spinal discs with prosthetic implants in the cervical region of the spine typically access the intervertebral space along an anterior-posterior direction, the surgeon often times removes the anterior longitudinal ligament in order to gain access to the intervertebral space. In some cases, the surgeon may also remove the posterior longitudinal ligament, for instance if it is desired to analyze possible impingements on the spinal cord. Removal of these ligaments eliminates one or more sources that promote normal anatomical motion between adjacent vertebrae.